ABSTRACT: Pharmacokinetics and pharmacodynamics of different drugs in adults and children are particularly heterogeneous in the latter. The differences between pharmacokinetics and pharmacodynamics prove unique studies but give rise to many ethical and practical issues. The main problems to be overcome when conducting clinical studies in children are the impact of the procedures and problems in patient recruitment. In general, the associated pain/anxiety and blood failure preclude pharmacokinetic studies in children, especially infants and young children. Population-based approaches based on pharmacokinetic-pharmacodynamic models are particularly attractive in children because these models can produce heterogeneous data. Quantitative/qualitative evaluation of the effects of studies on dose mixing and social interactions in societies and factors that may explain individual differences were noted.